NeoNET would have the most history in falling as JRJ international went from itself, to Fuchi, to Novatech to NeoNET.
That said the shakiest Mega to me has always been Wuxing, since it was only made AAA through a pile of bequests and taking advantage of a number of gifts + whatever geomantic shenanigans they have. Granted they may have seemed to suddenly rise to power due to lots of previous behind the scene manipulation so who knows.
Horizon, the other new kid on the block, seems too proficient with spin to fall from public opinion alone, much like the Azzies.
And again as Wyrm said, the only way any of the 'founding 7' can possibly fall per Corporate Court rules is if they are acquired by another corp. Ares, MCT, Shiawase and ORO/Aztechnology remain in their own hands. BMW --> Saeder-Krupp, Keruba International --> Renraku, JRJ International -->NeoNET have changed hands since 2023, but not often and the last permanent to change hands happened in 2059.
The last Mega to fall happened in 2064 when its CEO was erased by the Matrix Crash.